Foot Controller for Sewing Machine and Sewing Machine

ABSTRACT

A foot controller for a sewing machine includes a drive start switch ( 18 ) provided in a drive circuit ( 17 ) of a sewing machine motor ( 10 ) and a speed adjusting circuit ( 25 ) having a variable resistor ( 26 ) for adjusting a speed of the sewing machine motor ( 10 ) and a switching element ( 28 ) cooperating with the variable resistor ( 26 ), characterized by an interruption circuit ( 19 ) interrupting the drive circuit ( 17 ) of the sewing machine motor ( 10 ) when the switching element ( 28 ) is short-circuited while the drive start switch ( 18 ) is in an off-state.

TECHNICAL FIELD

The present invention relates to a foot controller for a sewing machine and a sewing machine, and more particularly to such a foot controller for a sewing machine which can detect and prevent a condition where failure of a foot controller conventionally results in abnormal rotation of a sewing machine motor, at a stage prior to start of sewing.

BACKGROUND ART

Various types of sewing machines such as embroidery sewing machines and lock-stitch sewing machines conventionally comprise a needlebar driving mechanism provided in a sewing arm for vertically moving a needlebar to which a sewing needle is attached and a thread loop taker such as a full-rotary hook provided in a sewing bed. When a user sets a sewing speed by a speed adjusting knob and then operates a start/stop switch, a sewing machine motor starts and the needlebar driving mechanism, thread loop taker and the like are driven via a main shaft of the sewing machine synchronously so that utility stitches, embroidery patterns and the like are formed on workpiece cloth placed on the bed at a previously adjusted sewing speed.

A foot controller is provided on the sewing machines in order that the user may instruct sewing start or sewing speed adjustment without use of his/her hands when sewing is carried out while the user is putting both his/her hands on the workpiece cloth to be sewn. In this case, a plug disposed on a distal end of a connecting cord extending from the foot controller is connected to a jack of a sewing machine body, the user can instruct sewing start by pressing the foot controller, and moreover, the sewing speed can be changed according to an amount of pressing.

For example, a safety gear described in patent document 1 is arranged so that when a user presses a pedal of a speed instructing unit, a resistance value of an internally provided variable resistor varies, and a speed control unit receives speed command voltage which is a speed command signal according to a resistance value of the variable resistor, and a sewing machine motor is controlled so that a sewing speed takes a value according to the speed command voltage.

The speed control unit controlling the sewing machine motor comprises a trigger control circuit, paired thyristors (silicon-controlled-rectifiers (SCRs)) or the like. Each thyristor is triggered when a pulse signal generated by the trigger control circuit based on a command signal from the variable resistor of the speed command unit is supplied to a gate thereof. AC current supplied from an AC power source is supplied via the paired thyristors to the sewing machine motor.

Patent document: JP-A-S55-71186

DISCLOSURE OF THE INVENTION Problem to be Overcome by the Invention

The electrical arrangement of the foot controller has recently been simplified. For example, as shown in FIG. 4, a speed adjusting circuit 125 provided in a foot controller 115 comprises a variable resistor 126, capacitor 127, resistance 129, diac 130, triac 128 and the like which are wired as shown in the figure. The foot controller 115 further comprises a drive circuit 117 and a drive start switch 118 closed when pressed by the user. The drive circuit 117 supplies current of an AC power source 111 to the sewing machine motor 110 in phase with trigger voltage applied to the gate of the triac 128, driving the motor 110.

When the triac 128 is normal, the drive start switch 118 is closed upon press of the foot controller 115 by the user. The trigger phase of impulses supplied from the diac 130 to the gate of the triac 128 is controlled based on a resistance value of the variable resistor 126 varied according to an amount of pressing. Since the phase-controlled current is supplied to the sewing machine motor 110, the motor is rotated at a speed according to the amount of pressing of the foot controller 115.

The above-described foot controller 115 has such a circuit arrangement as to be composed of the drive circuit 117 provided with only the drive start switch 118. Accordingly, in the case where the triac 128 fails thereby to be short-circuited, large current is supplied via the short-circuited triac 128 to the sewing machine motor 110 irrespective of phase control of the speed adjusting circuit 125 when the drive start switch 118 is closed concurrently with pressing of the foot controller 128 by the user. Consequently, the sewing machine motor 110 is rotated at high speeds concurrently with pressing of the foot controller 128 and furthermore, the speed control on the basis of an amount of pressing of the foot controller 115 is rendered ineffective.

The present invention was made in view of the foregoing circumstances and an object thereof is to prevent abnormal high-speed rotation of the sewing machine motor forcibly during start of sewing when a switching element provided in the foot controller has failed thereby to be short-circuited.

MEANS FOR OVERCOMING THE PROBLEM

A foot controller for a sewing machine of the present invention comprises a drive start switch (18) provided in a drive circuit (17) of a sewing machine motor (10) and a speed adjusting circuit (25) provided with a variable resistor (26) for adjusting a speed of the sewing machine motor (10) and a switching element (28) cooperating with the variable resistor (26), characterized by an interruption circuit (19) interrupting the drive circuit (17) of the sewing machine motor (10) when the switching element (28) is short-circuited while the drive start switch (18) is in an off-state.

Furthermore, a sewing machine which comprises a needlebar driving mechanism vertically moving a needlebar (5) having a lower end to which a sewing needle (6) is attached, a feed-dog vertically moving mechanism, a feed-dog horizontally moving mechanism, a thread loop taker accommodating a lower thread bobbin and cooperating with the sewing needle (6), a drive start switch (18) provided in a drive circuit of a sewing machine motor (10), and a foot controller (15) having a speed adjusting circuit (25) provided with a speed adjusting variable resistor (26) of the sewing machine motor (10) and a switching element (28) cooperating with the variable resistor (26), characterized in that the foot controller (15) includes an interruption circuit (19) interrupting the drive circuit (17) of the sewing machine motor (10) when the switching element (28) is short-circuited while the drive start switch (18) is in an off-state.

According to the above arrangement, the drive circuit (17) can be interrupted forcibly at a stage prior to start of sewing by the user when the switching element (28) fails thereby to be short-circuited. Accordingly, even when the user has pressed the foot controller (15), electric current is not supplied to the sewing machine motor (10) and consequently, high-speed rotation of the sewing machine motor (10) can reliably be prevented. Furthermore, since the sewing machine motor (10) is not driven even when the user presses the foot controller (15), failure of the foot controller can be recognized prior to sewing start by the user.

EFFECT OF THE INVENTION

According to the foot controller for sewing machine of the present invention or the sewing machine of the present invention, in the case where the switching element fails thereby to be short-circuited, high-speed rotation of the sewing machine motor can reliably be prevented even when the user has pressed the foot controller. Furthermore, failure of the foot controller can be recognized prior to sewing start by the user.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electronic sewing machine of an embodiment of the present invention;

FIG. 2 is a circuit diagram showing an inner circuit of a foot controller;

FIG. 3 is a view explaining phase control by a triac;

FIG. 4 is a view similar to FIG. 2, showing a prior art.

EXPLANATION OF REFERENCE SYMBOLS

-   -   M designates an electrically driven sewing machine, 10 a sewing         machine motor, 15 a foot controller, 17 a drive circuit, 18 a         drive start switch, 19 an interruption circuit, 20 a thermal         fuse, 21 electrical resistance, 25 a speed adjusting circuit, 26         a variable resistor, 28 a triac and 30 a diac.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described in the following with reference to FIGS. 1 to 3. Firstly, an electrically driven sewing machine M capable of sewing various utility stitches will be described in brief as shown in FIG. 1. The sewing machine M has a bed 1, a pillar 2 standing on a right end of the bed 1 and an arm 3 which extends from an upper end of the pillar 2 so as to be opposed to the bed 1. In the bed 1 are provided a feed dog vertically moving mechanism moving a feed dog vertically, a feed dog horizontally moving mechanism moving the feed dog horizontally, a thread loop taker (a horizontally rotating shuttle, for example) accommodating a lower thread bobbin and cooperating with a sewing needle 6, a thread cutting mechanism and the like (none of which are shown).

In the arm 3 are provided a needlebar driving mechanism vertically moving a needlebar 5 having a lower end to which the sewing needle (6) is attached, a needlebar swinging mechanism swinging the needlebar 5 in the direction perpendicular to a direction in which workpiece cloth is fed, a needle thread take-up driving mechanism moving a needle thread take-up vertically in synchronization with the vertical movement of the needlebar 5 and the like (none of which are shown). A sewing machine motor 10 is provided for driving via a main shaft of the sewing machine the feed dog vertically moving mechanism, feed-dog vertically moving mechanism, needlebar vertically moving mechanism and needlebar swinging mechanism.

A pattern display window 8 is provided in a front of the arm 3. Stitch patterns of various utility stitches are displayed on the pattern display window 8. A pattern selecting dial which is not shown is provided on a right side of the arm 3. The pattern selecting dial is turned so that a desired one of a plurality of types of patterns is selectable.

A foot controller 15 which can be pressed by a user is connectable via a connecting cord 16 to a jack provided in the right side of the pillar 2. Once the foot controller 15 is connected to the sewing machine M, the user can instruct start of a sewing operation only by pressing the foot controller 15 and moreover, a sewing speed can be changed according to an amount of pressing.

Next, the foot controller 15 will be described with further reference to FIG. 2. The foot controller 15 has a lower case 15 a and an upper case 15 b as shown in FIG. 1. The upper case 15 b has one end pivotally supported on the lower case 15 a and the other end which is vertically movable. A drive circuit 17, speed adjusting circuit 25 and the like as shown in FIG. 2 are arranged on a printed circuit board (not shown) provided in the lower case 15 a. When the user presses the upper case 15 b by his/her foot, the drive start switch 18 is turned on and a rotational speed of the sewing machine motor 10 is adjusted according to an amount of pressing.

The drive circuit 17 is provided with a first connecting line L1 connected directly to the sewing machine motor 10 and a second connecting line L2 connected via an AC power source 11 to the sewing machine motor 10. The drive start switch 18 and a thermal fuse 20 are provided for instructing to start drive of the sewing machine motor 10 and inserted in series to the first connecting line L1. Furthermore, electrical resistance 21 is inserted in parallel with the drive start switch 18.

The electrical resistance 21 is set at a relatively larger resistance value (1.8 KΩ, for example). An interruption circuit 19 comprises the electrical resistance 21, thermal fuse 20 and the like. The electrical resistance 21 and the thermal fuse 20 are integrally covered with cement while disposed in proximity to each other. More specifically, the interruption circuit 19 is arranged as a cement resistor with a built-in thermal fuse 20. The thermal fuse is adapted to be melted down when the electrical resistance 21 heats up to or above about 100° C., for example.

Furthermore, the resistance value of the electrical resistance 21 needs to be set at such a value, as maximum resistance value (at which electrical current is small and an amount of heat generated is rendered small), that an amount of heat sufficient to melt down the thermal fuse 20 is generated even when the ambient temperature is at 0° C. The resistance value of the electrical resistance 21 further needs to be set at such a value, as minimum resistance value (at which the sewing machine motor 10 is not rotated by current flowing through the electrical resistance 21 and an amount of heat does not reach Tm (maximum temperature limit) of the thermal fuse 20.

Tm is set at about 200° C. Furthermore, a temperature of the electrical resistance 21 in the melting of the thermal fuse 20 is set so as to be sufficiently larger than a melting temperature of the thermal fuse in view of time lag between generation of heat by the electrical resistance 21 and transfer of heat to the thermal fuse 20 or melting time of the thermal fuse 20.

When the resistance value of the electrical resistance 21 is excessively small, a temperature increased in the case of overcurrent becomes far higher than a melting temperature of the thermal fuse 20, exceeding Tm. As a result, the thermal fuse 20 has a possibility of re-conduction thereof after the melting. Accordingly, the resistance value of the electrical resistance 21 needs attention so as to be set in view of the current values during the normal and abnormal occasions, the melting conditions of the thermal fuse 20, drive conditions of the sewing machine motor 10.

The speed adjusting circuit 25 is provided with a third connecting line L3 connected to the first connecting line 1 and a fourth connecting line L4 connected to the second connecting line L2. Between the third and fourth connecting lines L3 and L4 is connected a parallel circuit of a triac 28 and a series circuit of a variable resistor 26 (maximum resistance value of 200 KΩ, for example) and a capacitor (0.1 μF) 27.

Furthermore, a series circuit of a resistance 29 (10Ω, for example) and a diac 30 is connected between a connection point P between the variable resistor 26 and the capacitor 27 and a gate G of the triac 28. When applied AC voltage exceeds a predetermined value, the diac 30 generates and delivers sharp impulses to the gate G of the triac 28.

Next, the operation of the foot controller 15 described above will be described. However, the resistance value of the variable resistor 26 takes a maximum value when the foot controller 15 is not pressed.

The drive start switch 18 is open when the user has not pressed the foot controller 15. However, a slight amount of current (no more than 1 mA) from the AC power source 11 flows to the drive circuit 17 via the parallel-connected electrical resistance 21. Such a small current results in a small amount of heat generated by the electrical resistance 21 and moreover, an amount of charge to the capacitor 27 is also small. As a result, the speed adjusting circuit 25 is not operated, whereupon the sewing machine motor 10 is not rotated.

When the user presses the foot controller 15 in order to start a sewing operation, the drive start switch 18 is switched to a closed state. In this case, current from the AC power source 11 flows through the drive start switch 18 of the first connecting line L1 and the thermal fuse 20, so that the capacitor 27 is further charged with the current.

When the terminal voltage of the capacitor 27 rises to or above a predetermined value, the diac 30 is rendered conductive such that sharp impulses are supplied from the diac 30 to the gate G of the triac 28. The triac 28 is then triggered to be operated. As a result, the AC current is supplied to a closed circuit of the first connecting line L1, third connecting line L3, triac 28, fourth connecting line L4 and second connecting line L2 sequentially, whereupon the sewing machine motor 10 is rotated.

Next, when an amount of pressing against the foot controller 15 by the user 15 is increased, the resistance value of the variable resistor 26 is reduced. Accordingly, a charging time for the capacitor 27 is shortened such that the time for trigger of the triac 28 is accelerated. Then, the time when the triac 28 is rendered conductive is accelerated, whereupon the current supplied to the sewing machine motor 10 is increased such that the rotational speed of the sewing machine motor 10 is increased. Thus, the resistance value of the variable resistor 26 is varied according to an amount of pressing against the foot controller 15 and the trigger time of the triac 28 is changed accordingly, whereby the phase control is carried out.

On the other hand, in a case where the triac 28 is short-circuited due to failure when the foot controller 15 is not pressed by the user, that is, when the drive start switch 18 is open, an amount of the current flowing through the electrical resistance 21 and thermal fuse 20 is increased to a maximum value and accordingly, an amount of heat generated by the electrical resistance 21 is increased. However, the AC current flowing in this case is insufficient to rotate the sewing machine motor 10. Since the electrical resistance 21 heats up to the maximum by this current, the thermal fuse 20 efficiently receives the heat generated by the electrical resistance 21.

As described above, an amount of heat generated by the electrical resistance 21 is increased, and the thermal fuse 20 provided in proximity to the electrical resistance 21 is heated in subjection to the heat generated by the electrical resistance 21. When the temperature of the thermal fuse 20 reaches a predetermined temperature (about 110° C., for example) or above, the thermal fuse 20 melts down. As a result, since the drive circuit 17 is interrupted by the interruption circuit 19, the AC current is not supplied to the sewing machine motor 10, and the drive circuit 17 can forcibly be interrupted at a stage prior to start of sewing by the pressing of the foot controller 15.

Accordingly, no current is supplied to the sewing machine motor 10 even when the user presses the foot controller 15 in order that sewing may be started, whereupon the sewing machine motor 10 can reliably be prevented from inadvertent high speed rotation. Furthermore, since the sewing machine motor 10 is not driven even when the user presses the foot controller 10, the user can recognize the failure of the foot controller 10 before start of sewing.

Furthermore, the interruption circuit 19 comprises the thermal fuse 20 connected in series to the drive circuit 17 and the electrical resistance 21 provided so as to be capable of heating the thermal fuse 10 and connected in parallel with the drive start switch 18. Accordingly, when the triac 28 serving as the switching element is short-circuited due to failure, the current from the drive current is supplied to the electrical resistance 21, which heats up. Since the thermal fuse 20 melts down by the resultant heat, only the provision of an inexpensive thermal fuse 20 can forcibly and reliably interrupt the drive circuit 17.

Furthermore, since the interruption circuit 19 comprises the cement resistor with built-in thermal fuse 20, the interruption circuit 19 can be rendered small-sized and accordingly, resistance to heat can be improved. Moreover, a work for mounting the interruption circuit 19 on the printed circuit board can markedly be simplified.

Furthermore, since the switching element comprises the triac 28 switched by the diac 30 connected to the variable resistor 26, the phase control of AC current can be realized by a simplified circuit arrangement while reduction in size of the switching element is achieved.

The present invention can be modified or expanded as follows:

1) Various switching elements such as a silicon controlled rectifier (SCR; or thyristor), field effect transistor (FET) or the like can be employed, instead of the triac 28.

2) When the cement resistor with built-in thermal fuse 20 is used as the interruption circuit 19, the resistance value of the electrical resistance 21, the distance between the electrical resistance 21 and the thermal fuse 20, the meltdown temperature of the thermal fuse 20 and the like may be changed according to the magnitude of AC current flowing into the drive circuit 17, a type of the cement resistor, a type of the foot controller 16 or the like. In short, the temperature of heat generated by the electrical resistance 21 and the meltdown time of the thermal use 20 may be changed arbitrarily.

3) The present invention should not be limited to the foregoing description of the embodiment and may be taken into practice by adding various modifications to the foregoing embodiment without departing from the gist of the invention. Accordingly, the invention involves these modified forms.

INDUSTRIAL APPLICABILITY

As obvious from the foregoing, the present invention is applicable to a foot controller for controlling a rotational speed of a sewing machine motor. 

1. A foot controller for a sewing machine, which comprises a start switch (18) provided in a drive circuit (17) of a sewing machine motor (10) and a speed adjusting circuit (25) provided with a variable resistor (26) for adjusting a speed of the sewing machine motor (10) and a switching element (28) cooperating with the variable resistor (26), characterized by an interruption circuit (19) interrupting the drive circuit (17) of the sewing machine motor (10) when the switching element (28) is short-circuited while the drive start switch (18) is in an off-state.
 2. The foot controller according to claim 1, wherein the interruption circuit (19) has a thermal fuse (20) connected in series to the drive circuit (17) of the sewing machine motor (10) and an electrical resistance (21) provided to be capable of applying heat to the thermal fuse (20) and connected in parallel with the drive start switch (18).
 3. The foot controller according to claim 2, wherein the interruption circuit (19) comprises a cement resistor (19) with the built-in thermal fuse (20).
 4. The foot controller according to claim 1, wherein the switching element (28) comprises a triac (28) switched by a diac (30) connected to the variable resistor (26).
 5. A sewing machine which comprises a needlebar driving mechanism vertically moving a needlebar (5) having a lower end to which a sewing needle (6) is attached, a feed-dog vertically moving mechanism, a feed-dog horizontally moving mechanism, a thread loop taker accommodating a lower thread bobbin and cooperating with the sewing needle (6), a drive start switch (18) provided in a drive circuit of a sewing machine motor (10), and a foot controller (15) having a speed adjusting circuit (25) provided with a speed adjusting variable resistor (26) of the sewing machine motor (10) and a switching element (28) cooperating with the variable resistor (26), characterized in that the foot controller (15) includes an interruption circuit (19) interrupting the drive circuit (17) of the sewing machine motor (10) when the switching element (28) is short-circuited while the drive start switch (18) is in an off-state.
 6. The foot controller according to claim 2, wherein the switching element (28) comprises a triac (28) switched by a diac (30) connected to the variable resistor (26).
 7. The foot controller according to claim 3, wherein the switching element (28) comprises a triac (28) switched by a diac (30) connected to the variable resistor (26). 